Our primary intent is to understand the DNA inversion system of bacteriophage Mu. A 3000 nucleotide pair region known as the G-segment is bordered by inverted repetitious sequences at which intramolecular recombination occurs to invert the G DNA segment. At least part of the trans-acting system necessary for the inversion is encoded within the beta region, adjacent to the G-segment. We wish: (a) to map the components of the inversion system; (b) to determine whether the sequence and/or the orientation of the G-segment is correlated with the plaque-forming ability of Mu; (c) to map the gene for the DNA modification function of Mu (mom); (d) to examine the possibility that DNA inversion (of the G segment) serves to control gene expression (of, for instance, the modification function); and (e) to compare the sequence and functions of the apparently identical G DNA segment of phages Mu and P1. To achieve these goals we shall isolate mutants with deletions, insertions, substitutions or duplications in the G-beta region of Mu. Some will be recovered from induced lysogens of Mu, using a selection based on the capacity of the capsid for packaging DNA. Others will be obtained by infection of E. coli. The mutants will be characterized by EM-heteroduplex methods and by restriction endonuclease digestion combined with slab gel electrophoresis. The physical maps of the mutant chromosomes will then be correlated with the observed biological properties.